The present invention refers to a method for producing a pore-containing opaque quartz glass.
Opaque quartz glass contains small bubbles that produce light scattering and give the glass a translucent to whitish appearance. There are smooth transitions between opacity and translucency. Translucency designates the partial light transmission which is not so much based on optical absorption as on light scattering. Opacity is the reciprocal property of translucency. In this respect, opacity is a measure of the light impermeability of quartz glass.
Silica raw materials of natural or synthetic origin are used for producing opaque quartz glass. Those of synthetic origin are typically produced from organic silicon compounds that are converted by polymerization, polycondensation, precipitation or CVD methods into SiO2. Fine dust particles of synthetic SiO2, which are often called “SiO2 primary particles”, are here obtained as an intermediate product, end product or by-product.
In the sol-gel method, the typical mean particle sizes of these SiO2 primary particles are in the range between 0.5 μm and 5 μm and are less than 0.2 μm in the case of flame hydrolysis. These are also called “SiO2 nanoparticles” or “SiO2 soot” in the technical literature. Due to their small particle size, their high specific surface area and low density, these primary particles are not free-flowing and show strong shrinkage when directly used in quartz glass production. As a rule, they are therefore granulated and pre-densified in advance.
Examples of suitable build-up or press granulation methods are roll granulation in a pan granulator, spray granulation, centrifugal atomization, fluidized-bed granulation, freeze granulation, granulation methods using a granulating mill, compaction, roller presses, briquetting, flake production, or extrusion.
During granulation discrete and rather large agglomerates, which are here called “SiO2 granulate particles,” or “granulate particles” for short, are formed by agglomeration of the SiO2 primary particles. In their entirety the granulate particles form a porous “SiO2 granulate”. The porous SiO2 granulate is free-flowing and has a bulk weight which is increased by comparison with the finely divided SiO2 dust and can be further increased by way of thermal or mechanical treatment.
A method for producing closed-pore opaque quartz glass is known from U.S. Pat. No. 4,042,361 A. The publication describes the manufacture of a quartz glass crucible of opaque quartz glass by way of a slip casting method using synthetic quartz glass granules. The quartz glass granules are made from pyrogenically produced SiO2 soot, as accrues as filter dust in the flame hydrolysis of a silicon compound, by preparing a gel from the loose SiO2 soot first by mixing into water and stirring, the gel having a solids content varying between 30% by wt. and 45% by wt., depending on the type and speed of the stirring process. The fragments obtained after drying of the gel are sintered at temperatures between 1150° C. and 1500° C. into dense, coarse quartz glass granules. These are subsequently fine-milled in a dispersion liquid into grain sizes between 1 μm and 10 μm. The slip produced thereby is cast into a crucible mold, and the layer adhering to the edge of the crucible is dried so as to form a porous green body. The green body is subsequently vitrified at a temperature between 1800° C. and 1900° C. into the quartz glass crucible.
The known method requires multiple method steps that partly entail a high expenditure of energy, such as for instance the vitrification of the coarse-grained gel fragments into the desired quartz glass granules. In the comminuting and milling processes, there is the fundamental risk of contamination of the milled material and of dispersion caused by abrasion from the milling tool, especially when the milled material has a high mechanical strength.
DE 102 43 953 A1 discloses a method for producing opaque quartz glass according to the above-mentioned type. As starting material, a SiO2 granulate of porous SiO2 granulate particles is employed that are formed from agglomerates of nanoscale, amorphous, synthetically produced SiO2 primary particles having a mean primary particle size of less than 100 mm.
To produce the SiO2 granulate, a roll granulation method is employed, resulting in granulate particles with particles in the range between 100 μm and 500 μm and with a specific BET surface area of about 45 m2/g. Thermally consolidated porous “fine granulate” with a specific BET surface area of about 30 m2/g and a tamped density of about 1.3 g/cm3 is produced from this raw granulate by treatment in a rotary furnace at a temperature of 1200° C. in chlorine-containing atmosphere. By heating to an elevated temperature of 1450° C., a further part of the raw granulate is completely vitrified into synthetic quartz glass granules having a specific BET surface area of about 1 m2/g. The particles of the fine granulate and of the quartz glass granules have a mean size (median or D50 value) of less than 160 μm. The D50 value represents the particle size that is not reached by 50% of the cumulative particle volume.
A preparation of a 50:50 mixture of SiO2 fine granulate and quartz glass granules is stirred into deionized water, whereby a liter weight of the dispersion of 1.6 kg/l is set. In a ball mill lined with polyurethane, the dispersion is homogenized for about one hour and is subsequently cast into a porous plastic mold in which dehydration and body formation take place to form an open-pore green body. The drying process already yields a firm bond between the individual granulate particles and leads to a densification and consolidation of the green body, which is intended to facilitate subsequent sintering.
DE 103 44 189 A1 describes a method for producing a cast part of opaque quartz glass by way of a slip casting method. The SiO2 slip is cast into a liquid-impermeable mold and a frozen blue body is first formed, the blue body being subsequently dried and sintered. The mold may be a membrane mold of vacuum-shaped silicone.
The granulation of SiO2 primary particles normally yields an undesired fine fraction with granulate which is not agglomerated or is insufficiently agglomerated. This fraction causes problems in the subsequent processing of the granulate. This drawback is avoided by spray granulation, as is for instance known from EP 1 148 035 A2. The predetermined size of the granulate particles can here be set relatively precisely by separating the fine fraction with a diameter below 100 μm directly in the granulation process by means of a cyclone.
Spray granulate particles have a spherical morphology and are present as an agglomerate of individual spherical SiO2 primary particles that can be crushed and comminuted by applying a slight mechanical pressure. They typically have a hollow space and a mean grain diameter in the range of 100 μm to 300 μm. The specific surface (according to BET) of spray granulate is normally around 30 m2/g. The process-specific hollow-space formation and the low densification in the spray granulate lead during slip formation to a thickening of the slip as the moisture bound in the hollow spaces and in the porosity of the granulates must be compensated. The accompanying risk of lump formation is avoided by low solids contents of the slip.
On the other hand, spray granulation allows inexpensive production together with high reproducibility of the properties of the spray granulate, especially of the diameter of the granulate particles. It would therefore be desirable to avoid such drawbacks.
It is therefore an objective of the present invention to provide a procedure for producing opaque quartz glass by way of slip casting that is also suited for the employment of spray granulate.